There is an episode about the December holidays, Christmas, Hanukkah (yes, I know it can be in November), and Kwanzaa. It is revealed in this episode that Sid's mom grew up celebrating Christmas and Kwanzaa, leading most viewers to assume she is African-American and that his dad celebrated Hanukkah. On the Sid the Science Kid wiki page (see the related link below) this theory is verified.
There is nothing on the site about May, Gerald and Gabriella, but my husband and I assume May is probably Chinese because she mentions eating scallion pancakes at home and her cat is named Mooshu. I believe most people assume Gabriella is Hispanic and Gerald is Caucasian.
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He is mixed black and caucasian/Jewish.
"Sid's mother is of African descent and his father grew up Jewish as noted in the Hanukkah/Christmas/Kwanzaa episode."
-Wikipedia
Sid the Science Kid - 2008 Seed the Science Kid 2-21 was released on: USA: 25 January 2012
Sid the Science Kid - 2008 Must See TV 1-19 was released on: USA: 23 October 2008
Sid the Science Kid - 2008 Sid's Special Dad Day - 1.32 was released on: USA: 15 September 2009
Sid the Science Kid - 2008 Sid's Special Dad Day 1-32 is rated/received certificates of: Australia:G
Sid the Science Kid - 2008 The Rolie Polie 1-2 is rated/received certificates of: Australia:G
The duration of Sid the Science Kid is 1800.0 seconds.
Sid the Science Kid was created on 2008-09-01.
Sid the Science Kid - 2008 Sid's Pet Project - 2.19 was released on: USA: 2011
Sid the Science Kid - 2008 Seed the Science Kid 2-21 was released on: USA: 25 January 2012
Sid the Science Kid - 2008 was released on: USA: 8 September 2008
Sid the Science Kid - 2008 Sid the Weatherman 1-35 is rated/received certificates of: Australia:G
Sid the Science Kid - 2008 Sid the Weatherman 1-35 was released on: USA: 18 September 2009
Sid the Science Kid - 2008 Sid's Amazing Lungs - 1.38 was released on: USA: 18 May 2009
Sid the Science Kid - 2008 is rated/received certificates of: Australia:G USA:TV-Y
Sid the Science Kid - 2008 Sid's Health Day 1-20 is rated/received certificates of: Australia:G
Sid the Science Kid - 2008 Sid's Super Kick 2-5 is rated/received certificates of: Australia:G
Sid the Science Kid - 2008 Sid's Holiday Adventure 1-33 is rated/received certificates of: Australia:G